There are two general categories of infrared sensors (hereinafter referred to as “IR sensor”), namely, a cooled type IR sensor and an un-cooled type IR sensor. The cooled type IR sensor detects electric signals generated by interactions between photons of infrared ray and electrons of an object. The un-cooled type IR sensor operates by detecting thermal variation created by the infrared absorbed in the object. The cooled type IR sensor mainly comprises semiconductor devices and provides low noises and a quick response time. There are, however, several shortcomings in the cooled type IR sensor. For example, liquid nitrogen temperature, −193° C., is required to activate the cooled type IR sensor. The un-cooled IR sensor has a relatively low performance, but can operate in a normal temperature. Thus, the cooled IR sensor requiring a cooling process is mainly used in a munitions industry. On the other hand, the un-cooled IR sensor is mainly produced for civilian purpose.
The un-cooled type IR sensor is again divided into a bolometer type, a thermocouple type, and a pyro-electric type. The pyro-electric type IR sensor has high detectivity but low productivity. In contrast to the pyro-electric type IR sensor, both the bolometer type and the thermocouple type IR sensors have a relatively lower detectivity than that of the pyro-electric type IR sensor, but high productivity can be achieved by manufacturing the IR sensors together with detection circuits on a single silicon wafer (monolithic type). Therefore, both the bolometer type and the thermocouple type IR sensors meet the civilian demands. The bolometer type IR sensor detects infrared ray by absorbing infrared ray radiated from an object, converting the absorbed infrared into thermal energy causing temperature increase, and measuring the resistance variation generated by such thermal variations.
FIG. 1 illustrates a two-level bolometric IR sensor as disclosed in U.S. Pat. No. 5,300,915 referred to as “thermal sensor”.
Referring to FIG. 1, the two-level bolometric IR sensor 10 comprises a raised upper layer 11 and a lower layer 12. The lower layer has a flat surface semiconductor substrate such as a silicon substrate. The surface of the silicon substrate has several components of an integrated circuit including diodes, x and y bus lines, connections, and contact pads at the end of the x bus lines. The raised upper layer 11 includes a first silicon nitride layer, a bolometer layer, a second silicon nitride layer therebetween, and an IR absorbing layer over the second nitride layer. The two layers 11 and 12 are separated by a cavity.
In the above-mentioned prior IR sensor, a number of problems exist. For example, a plural of supporting parts are placed on the raised upper layer, thereby decreasing an entire area for absorbing the infrared ray. Therefore, a maximum area for absorbing the infrared ray can be hardly achieved.
Korean Patent No. 10-299642, Korean Patent No. 10-299643, U.S. Pat. No. 6,441,374, and U.S. Pat. No. 6,448,557 disclose IR sensors and fabricating methods thereof for improving sensitivity and fill factor of an IR sensor, such as a bolometric sensor with three layers, a method for manufacturing a bolometric sensor with three layers, a bolometric sensor with three layers including a infrared reflective layer, and a thermal type infrared ray detector with a thermal separation structure, respectively.
However, in the above-mentioned prior art, electrical and structural properties of infrared sensors are used to improve an absorption rate. Thus, in comparison to IR sensors designed based on a spectroscopic approach, such prior IR sensors provide only a relatively low absorption rate. In addition, in the infrared ray absorbing bolometer according to the prior art methods, an upper part absorbing an infrared ray is raised from an bottom part (i.e., cavity is formed), thereby resulting in the deformation of the upper part of the IR sensor, detrimentally affecting the characteristics of the IR sensor.
To solve such problems, several solutions have been provided. For example, in Korean Publication No. 2000-46515, an IR sensor with three layers is disclosed which comprises a silicon oxy-nitride layer both on and under a silicon oxide layer again enclosing a bolometer layer to prevent the deformation of the IR sensor caused by reactions between vapors in the air and the silicon oxide layer. In Korean Publication No. 2000-04158, an bolometric IR sensor is disclosed which comprises a bolometric IR sensor is disclosed which comprises a driving layer, a supporting layer, a backing layer therebetween and an absorbing layer.
In spite of such solutions, there still exist a number of shortcomings. In detail, additional processes are required to form the silicon oxy-nitride layer and heat, instead of vapor, still cause the deformation of the upper part. In addition, the formation of the backing layer also requires additional processes and reduces an IR absorbing area.